To transport a deployable antenna mounted on an artificial satellite or the like to space, a rocket such as an Ariane rocket, or H-IIA is used as a transporting unit. However, the payload shroud of a rocket is restricted. Therefore, to transport a large deployable antenna, it is accommodated in a rocket in a small, folded state and is deployed after the rocket reaches space. Among deployable reflectors constituting such a deployable antenna, exemplified is one that is a combination of plural fundamental structures and can form a large deployable antenna depending on the number of fundamental structures.
FIG. 13 shows the configuration of a conventional deployable reflector in which part (a) is a perspective view showing the entire configuration of the deployable reflector and part (b) is an exploded perspective view showing an exemplary fundamental structure of the deployable reflector. As shown in the figure, the fundamental structure of the deployable reflector is composed of an integrated system of cables 100 with metallic meshes, stand-offs 105, and a deployable truss structure 106. Attached to the deployable truss structure 106 via the stand-offs 105, the integrated system of cables 100 with metallic meshes assumes a polyhedron so that the surface of the deployable reflector approximates a parabolic shape. The deployable truss structure 106 is deployable and foldable. The deployable truss structure 106 in a deployed state keeps the stand-offs 105 at predetermined positions, whereby the integrated system of cables 100 is rendered in a tense state and a predetermined parabolic shape is formed.
FIG. 14 is an exploded perspective view showing the details of the integrated system of cables 100. As shown in the figure, the integrated system of cables is decomposed into a surface cable system 101, metallic meshes 102, tie cables 103, and a back cable system 104.
The metallic meshes 102 are attached to the surface cable system 101 and the individual connecting points of the surface cable system 101 are lowered (as viewed in the figure) by the tie cables 103, whereby a predetermined polyhedral shape is formed. To give tension to the tie cables 103, the back cable system 104 is disposed on the opposite side of the tie cables 103 to the surface cable system 101.
To obtain a predetermined surface shape, during manufacturing process, the cable lengths are adjusted while the shape of the surface cable system 101 is measured, whereby a predetermined surface shape is obtained when the stand-offs 105 are located at predetermined positions.
FIG. 15 illustrates a procedure for forming a surface shape with predetermined accuracy in manufacturing process of the conventional deployable reflector. As shown in the figure, to obtain a predetermined surface shape in manufacturing process, work of adjusting the cable lengths of the tie cables 103 while measuring the shape in a state that a reflector surface is stretched by giving tension to the surface cable system 101 and the back cable system 104 so that they are pulled outward is repeated until the surface shape is formed with predetermined accuracy. For example, if part of the surface cable system 101 is deviated upward from the intended parabolic surface when a shape is measured during manufacturing process, that part of the surface cable system 101 is lowered by shortening related tie cables 103, whereby the shape is changed to come closer to the predetermined shape. The shape is determined by the tensile states of the cables, and the tensile states vary when a certain cable is shortened. Therefore, the predetermined shape cannot be obtained merely by a single adjustment. In view of this, work of measuring deviations by a shape measurement and adjusting the lengths of the tie cables 103 for individual connecting points 110 is repeated, whereby the cable sections of the surface cable system 101 are adjusted so as to be located at predetermined positions and the predetermined shape is achieved.
To facilitate adjustments, the tie cables 103 extend in the direction that traverses the surface cable system 101 so that adjustment of each tie cable length causes a large variation at its connecting point in the surface shape. The surface cable system 101 is made of a cable that is low in stiffness and is relatively large in the ratio of the length variation to the tension variation, and the back cable system 104 is made of a cable that is high in stiffness and is small in the ratio of the length variation to the tension variation. As a result, as shown in FIG. 15, when the length of a tie cable 103 is changed, the positions of the surface cable system 101 connected to the tie cable 103 are mainly changed.
FIG. 16 is a perspective view showing the configuration of another conventional deployable reflector. In the figure, an integrated system of cables 201 is made of a cable that is high in stiffness and is small in the ratio of the length variation to the tension variation, and is supported by support cables 202 that are low in stiffness and is relatively large in the ratio of the length variation to the tension variation. The support cables 202 are attached to an inflatable membrane 203, and the integrated system of cables 201 is rendered tense in a state that the inflatable membrane 203 is expanded by, for example, injunction of air.
FIG. 17 is a perspective view showing the configuration of a further conventional deployable reflector. FIG. 18 is an exploded perspective view showing individual components of this conventional deployable reflector. As shown in the figures, the deployable reflector is composed of an integrated system of cables 100 that functions as an antenna reflection surface and a deployable truss structure 106 as a frame structure. The integrated system of cables 100 is composed of a surface cable system 101, metallic meshes 102, tie cables 103, and a back cable system 104, and is supported by the deployable truss structure 106 via plural stand-offs 105.
The deployable truss structure 106 is composed of eight planar linkages 107 each of which assumes a trapezoidal shape. The planar linkages 107 share a central shaft member 108 and are disposed radially around the central shaft member 108 so as to form the same angles. The deployable truss structure 106 can be folded or deployed by sliding a slide hinge 109 in the axial direction of the central shaft member 108 (disclosed in “A Modular Cable-Mesh Deployable Structure for Large Scale Satellite Communication Antennas”, by Akira Meguro, Jin Mitsugi, and Kazuhide Ando, The Institute of Electronics, Information and Communication Engineers B-II Fascicle “Small Special Issue on Next-generation Satellite Communication Technologies,” The transactions of the Institute of Electronics, Information and Communication Engineers, B-II, Vol. j76-B-II, No. 5, 1993, pp. 476–484).
For expansion/contraction driving of the planar linkages, annular cables are used that are disposed around the deployable truss structure. Movably connected to tip portions of the respective planar linkages, these cables extend or contract the planar linkages in a synchronized manner by adjusting the cable take-up lengths by rotation of a motor.
Incidentally, the integrated system of cables 100 of the conventional deployable reflectors is made of a pliable, flexible cable and cannot maintain an antenna reflection surface shape on its own. Therefore, the top plane of the deployable truss structure 106 is formed into an approximated spherical shape that approximates a parabolic surface with minimum errors and the integrated system of cables 100 is attached to the deployable truss structure 106 from above via the stand-offs 105, whereby a parabolic surface shape of the integrated system of cables 100 is maintained.
However, with this method, the shape of the integrated system of cables 100 strongly depends on the shape of the deployable truss structure 106. Therefore, for the integrated system of cables 100 to have a highly accurate reflector surface, it is necessary that the deployable truss structure 106 be rigid enough to sustain the tension of the integrated system of cables 100. However, to increase the stiffness of the deployable truss structure 106, it is necessary to make the individual members constituting the deployable truss thicker, which raises a problem of increase in total weight.
Further, in the conventional deployable reflectors, the positions of the stand-offs 105 which supports the integrated system of cables 100 by giving tension thereto vary due to such factors as thermal distortion of the deployable truss structure 106 and repeatability of its deployed shape. As a result, the balance state of the integrated system of cables 100 varies and lengths of the individual cable sections and therefore the shapes vary. The sensitivity to deformation of the support portion is thus high, and hence it is necessary to form the support structure with high accuracy. In particular, since the back cable system is made of a cable that is high in stiffness and is small in the ratio of the length variation to the tension variation from the viewpoint of ease of adjustment, slight variations in the positions of the stand-offs cause large tension variations. The tension variations of the individual cable sections in a balanced state cause length variations of the surface cable system, resulting in a problem that the surface shape of the surface cable system varies greatly.
On the other hand, in the conventional deployable reflector using the inflatable membrane, the inflatable membrane is made of a thin film so as to be deployable and foldable. Therefore, the inflatable membrane is easily deformed by the tension for stretching the reflector surface. Further, it is very difficult to predict behavior of a film-like structure and hence it is difficult to predict positional deviations with respect to a distant antenna feeder. This means a problem that it is difficult to allow the conventional deployable reflector to function as a reflector.
In particular, where a reflector is constructed by using an inflatable membrane, the positions of the reflector surface are not determined correctly in a state that the membrane is expanded. Further, since there is influence of gravity on the ground, it is important to predict a shape analytically taking the zero-gravity state in satellite orbit and other factors into consideration. However, according to the current analysis techniques, it is very difficult to predict a membrane shape with high accuracy. In particular, although the deployable reflector itself needs to be supported in a certain way, a problem remains that when the membrane is supported it is difficult to predict a positional relationship between the support positions and the reflector surface.
An object of the present invention is to realize a deployable reflector that is lighter and larger than conventional ones as well as a deployable reflector in which the deformation sensitivity of a reflector surface to displacements of support positions of a deployable truss structure that supports an integrated system of cables by giving tension thereto can be lowered and bending moment that is generated to give tension to the integrated system of cables and acts on the deployable truss structure can be reduced.